The present invention relates to an improved coinjection nozzle for injection molding and to an improved injection molding method.
Hot runner injection nozzles having torpedoes or valve stems in the melt stream typically create weld line blemishes in the finished part caused by the melt stream being divided by these obstructions and having to reform downstream thereof. The present invention provides an improved injection nozzle and method which includes an improved flow channel geometry to eliminate or significantly minimize these weld lines, while at the same time permitting faster color change performance.
When plastic melt flows through a hot runner system on route to a mold cavity, it sometimes must separate from a single solid cylindrical flow mass to pass by obstructions, such as torpedos, support fins and blades, valve stems, stem guides or support blades or other melt ducts or nozzles, etc. When thus divided, the disturbed melt stream recombines downstream of the obstruction and there forms at least one weld line as the melt streams from the divergent paths come back together. Such a weld line, unless remixed homogeneously, thereafter continues to be present, in the melt stream and appears as a blemish or line in the molded part formed in the mold cavity. This may also result in preferential flow and other problems. Also when changing color of the melt considerable amounts of resin are wasted in flushing out the old color that is caught or stuck to these flow obstructions.
In coinjection, different melt streams are passed through the injection nozzle to make a multimaterial or multilayered part or preform. Thus, the coinjection nozzle effectively consists of several nozzles assembled coaxially to form annular spaces therebetween to run the various materials that make a coinjection molded part. These annular spaces are usually fed by one or more portals. When each material or melt is fed into one of the annular spaces at least one weld line is formed in each melt stream. In addition, a preferential flow occurs on the side that the nozzle is fed from. A weld line and a preferential flow are also generated when the melt flows around a valve stem. The resultant weld lines are then transferred to the resultant part and the preferential flow induces what is known in preform coinjection as a xe2x80x9cdipxe2x80x9d in the neck of the preform, for example. The weld lines are weaknesses in the part that may induce its premature failure or its failure at low stress loading conditions. In the case of preform molding, the presence of a weld line may induce part failures during blow molding, or failure of a blow molded bottle at lower stress loading conditions. In coinjection molded preforms, a dip or unsymmetrical distribution of an intermediate resin layer, which may be generated by the weld lines and preferential flow, is a significant defect of coinjection molded preforms as it represents an area that does not have the same layered structure as the rest of the preform and therefore has poor properties, for example, barrier and mechanical properties. The dip may also represent a visual defect in the preforms or final part.
It is, therefore, a principal object of the present invention to provide an improved coinjection nozzle and method.
It is a still further object of the present invention to provide an improved coinjection nozzle and method as aforesaid which eliminates or significantly minimizes weld lines and dips in the molded article, and also to provide an improved method of simultaneous coinjection of two or more materials.
Further objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention the foregoing objects and advantages arc readily obtained.
The coinjection nozzle of the present invention comprises: a first flow channel for a first resin flow communicating with an outlet area for transferring molten first resin to a mold cavity, said first flow channel including an outer surface thereof, an elongated shaft extending in the first flow channel adjacent the outlet area, at least one first spiral groove formed in the outer surface of the first flow channel and facing the shaft that decreases in depth towards the outlet area, with lands adjacent said first groove that increase in clearance towards the outlet area, wherein a helical flow path of said first resin is provided through the first spiral groove and an axial flow path of said first resin is provided over the lands; and at least one second flow channel for a second resin flow communicating with said outlet area, at least one second spiral groove in the second flow channel that decreases in depth towards the outlet area with lands adjacent said second groove that increase in clearance towards the outlet area, wherein a helical flow path of said second resin is provided through the second spiral groove and an axial flow path of said second resin is provided over the lands.
The coinjection method of the present invention comprises: supplying a first molten resin to a first flow channel having an outer surface thereof, in an injection nozzle, which first flow channel extends in said nozzle to an outlet area for transferring said first me resin to a mold cavity, providing an elongated shaft in said first flow channel adjacent said outlet area, transferring said first molten resin to at least one first spiral groove, with lands adjacent said first groove, said first groove formed in the outer surface of said first flow channel and transferring said first resin from said first groove to said outlet area, decreasing the depth of said first groove toward the outlet area and increasing the clearance of said lands towards the outlet area, thereby flowing said first resin in a helical flow path through the first spiral groove and in an axial flow path over the lands; and supplying a second molten resin to a second flow channel in said injection nozzle, which second flow channel communicates with said outlet area, transferring said second molten resin to at least one second spiral groove in said second flow channel with lands adjacent said second groove, and transferring said second resin from said second groove to said outlet area, decreasing the depth of said second groove towards the outlet area and increasing the clearance of said lands towards the outlet area, thereby flowing said second resin in a helical flow path through the second spiral groove and in an axial flow path over the lands.
Further features of the present invention will appear hereinbelow.